Nitrogen gas has many useful industrial applications. For example, it can be used to clean or purge piping systems, chemical plants, or pipelines. Furthermore, nitrogen is clean and can be economically produced. The use of nitrogen is often more advantageous than air since it is denser than air and therefore requires less volume for storage and pressurization purposes. In addition, where there is a danger of fire, nitrogen is preferred over air because it is non-flammable.
The use of nitrogen is particularly important in the oil and gas industry. For example, nitrogen is commonly used in drilling operations for aerating the mud which may be encountered in drilling and in cleaning out the drill stem itself. Nitrogen may also be used in coil tubing work for cleaning out the parafin or other materials which may plug older wells. In order to check drill pipes or well heads for leaks, nitrogen is frequently used to pressurize these devices.
Nitrogen is also directly utilized in the recovery of oil and gas. By a process commonly referred to as "fracturing", the reserves of a new or nearly-depleted oil or gas well can be economically recovered. Under this process, nitrogen and other materials are injected at high pressures into the well in order to force apart the various strata of earth and rock surrounding the well. The oil and/or gas which is trapped within these fractured strata is then forced or permitted to flow by gravity to a well where it can be efficiently pumped or otherwise brought to the surface.
In this fracturing process the nitrogen is typically trucked to the well site in a cyrogenic or liquid state. For offshore drilling operations, the nitrogen may be transported to the well site by barges or boats. The liquid nitrogen is then pumped at a very high pressure into a vaporizer mounted on the transporting vehicle. The vaporizer heats the liquid until it assumes a gaseous state, whereupon the pressurized nitrogen gas is forced into the well in order to accomplish its fracturing function.
The liquid nitrogen is pressurized by large pumps which are also mounted on the transporting trucks. These pumps are specially designed and constructed to withstand the extreme cold temperatures, of liquid nitrogen, which may be as low as 300-400 degrees Farenheit (F.) below zero. Such cryogenic pumps generally comprise an assembly of several pumping cylinders, each having a displacement type piston, valves, and inlet and outlet ports. Thus, for example, each pump may have one or as many as 5 or 6 cylinders, with each cylinder comprising, in effect, an individual pump.
Typically, a "triplex", or three-cylinder pump, is arranged on each side of truck. In operation, all of the cylinders on one side of the truck are simultaneously pumping in parallel. Thus, it is common for the nitrogen to reach a flow rate of more than 100 gallons per minute and pressures of 10,000 to 15,000 pounds per square inch gauge (p.s.i.g.). There are times, however, when the flow rate of the nitrogen must be reduced to such a small amount that it cannot be accomplished by simply reducing the speed of the pump. Under these conditions, it is advantageous to deactivate two of the three cylinders of the pump so that only a single cylinder is operating, thereby permitting more accurate control over the reduced flow of the nitrogen into the well. In the past, this deactivation has been accomplished by diverting the nitrogen flowing from two of the cylinders back into the tank on the truck. Valves on the diversion conduits are opened in order to permit this return flow of the nitrogen to truck, and check valves are utilized to prevent the back-flow from the single operating cylinder into the diversion system.
This type of diversion system, however, suffers from several disadvantages. First, it is quite costly. The diversion conduits and valves are expensive to build and to install. Secondly, the space available on the nitrogen transporting trucks is very limited, due to the large equipment mounted on them and the numerous liquid and electrical conduits. Such a diversion system is bulkly and occupies valuable space that could be more efficiently and economically utilized to accomodate a larger nitrogen tank. Furthermore, the valves and check valves of the diversion system have a limited life and soon develop leaks, especially when subjected to the rapid cycling of the liquid nitrogen flow.
Finally, the return flow of nitrogen back into the tank adds energy to the nitrogen already present. Often times, the nitrogen being pumped out of the tank is at a higher temperature than that in the tank, or it may even be partially or wholly vaporized. Thus, this return flow into the tank heats the existing liquid and often causes it to blow through the tank's relief valve, thereby wasting the nitrogen gas and adding to the cost of the diversion system.